International Journal of Geomechanics

This paper presents a precise solution to coupled thermohydromechanical problems of layered, transversely isotropic, saturated, porous media with a heat source or a temperature increment with the aid of integral transform techniques. On the basis of governing equations of three-dimensional (3D) coupled thermohydromechanical problems of saturated porous media, a standard differential matrix equation was deduced by using the Laplace-Fourier transform. In this paper, an extended precise-integration method for a layered system with an internal or surface load is introduced. It was used to solve the previous standard differential equation in the transformed domain. Real solutions in the physical domain were obtained by using the numerical inversion of the Laplace-Fourier transform. The precision of the presented theory was confirmed by two examples, and the effects of the anisotropic permeability and the transverse isotropic characteristics on the thermohydromechanical coupling behavior of layered saturated media are described.

Ground disturbance and deformation caused by a controlled gas release prior to engineering construction are commonly encountered geotechnical and geoenvironmental problems in shallow gas geology areas. In this work, typical characteristics of shallow gas geology and gas-charged sand in the Hangzhou Bay of China are analyzed. It is found that the occurrence configuration feature of sand in the shallow gas enrichment zone of reservoirs is similar to that of the common unsaturated soil, but its pore gas pressure is several times larger than the atmospheric pressure. The effect of the suction history on the deformation characteristics of sand is investigated by triaxial simulation tests using the Global Digital System (GDS). The deformation of reservoir sand caused solely by the suction reduction in the process of gas release is negligible, and the main deformation is attributed to the increasing net mean stress arising from the gas pressure reduction in the reservoir. Based on the findings from the triaxial tests, a simplified constitutive model for the gas-charged sand under the special stress path of gas release is developed to describe its deformation behaviors. Then, combined with the two-phase flow equation of incompatible fluid seepage, the proposed model is applied to the three-phase soil-water-gas–coupled analysis of a typical gas-bearing ground with a gas exhaust shaft. The numerical results match the field monitoring data well. Thus, the proposed method can be used to quantitatively evaluate the geoenvironmental effect of an engineered gas release on existing adjacent structures in shallow gas regions.

A theoretical study has been performed for evaluating maximum resultant active force exerted by a cohesive-frictional backfill against a rigid wall with inclined back face undergoing outward horizontal translational movement in the presence of pseudostatic seismic loadings. With the application of a new kinematically admissible translational mechanism in the context of the upper bound limit theorem of plasticity, seismic active thrust has been derived as a function of nondimensional seismic active earth thrust coefficients because of the contributions of soil unit weight, surcharge pressure, and cohesion of soil. The present analysis was performed by postulating a composite collapse mechanism comprised of a central radial shearing zone enclosed between triangular blocks at either side. The kinematically admissible velocity field of the radial shear zone freely varies in between the velocity field used for defining the conventional circular to log spiral shear zones available in the literature. The influence of seismic acceleration coefficients, location of surcharge pressure from the wall crest, backfill slope angles, characteristics of interface between soil and wall, orientations of wall, and properties of backfill on the magnitude of active earth force has thoroughly been examined.

Ground reinforcement using bolts is effective for underground excavations. The ground response of expansive loess tunnels with bolt reinforcement remains poorly understood. Therefore, this paper proposes an analytical solution for bolted rock in expansive ground using the convergence-confinement method. On the basis of the bilinear strength criterion for rock mass, a computational procedure was used to obtain the ground reaction curve (GRC) for expansive loess tunnels with bolt reinforcement. The proposed approach considers ground expansion stress and geometrical parameters, mechanical parameters, and pretension of bolts. The results confirm the positive influence of bolt reinforcement and the negative influence of expansion stress on the GRC. Bolt reinforcement offers good resistance against harmful expansion stress and improves the overall stability of loess tunnels. When the expansion stress of an expansive loess tunnel reaches a certain limit, bolt reinforcement is necessary for tunnel stability. This critical limiting value of the expansion stress can be evaluated using the proposed approach. The proposed analytical solution provides guidelines for the stability estimation of tunnel excavations in expansive ground.

The response of tunnel excavations in blocky rock mass is dominated by the presence of discontinuities. However, many of the studies reported in the literature consider these geological features in a simplistic manner by adopting the equivalent continuum approach, thereby neglecting the structurally controlled mechanism of response. To overcome the aforementioned limitation, the present study adopts the Voronoi tessellation scheme in a discrete-element-based framework to simulate the blocky rock mass. An attempt was made to frame the prediction model for convergence strains of the tunnel by considering the uncertainties of joint parameters and in situ stress ratio. The prediction model was framed following the identification of important parameters affecting the strains using the robust central composite design. Finally, the concept of critical strain was used to demonstrate the applicability of the framed model through the probabilistic assessment of various performance levels of tunnel excavation.

The elastic compliance matrix is a key parameter for the stability analysis of engineering projects constructed in fractured rock masses. In this study, the energy equivalence and the superposition methods were used to estimate the elastic compliance matrix of rock masses containing persistent or nonpersistent fractures. For the energy equivalence method, two loading schemes were proposed to obtain the values of $S_{16}$ and $S_{26}$ for nonsymmetric fracture sets. The superposition method was proposed to estimate the compliance matrix of rock mass containing several fracture sets by summing the matrices of intact rock and each single fracture set. For a rock mass containing regular persistent fracture sets, the derived analytical results were consistent with closed-form results. The analytical results were also compared with the results of FEM for a rock mass containing two normally distributed nonpersistent fracture sets. The maximum deviations of the directional Young’s modulus, shear modulus, and Poisson’s ratio were 7.9, 11.3, and 21.3% respectively. The limitations of the proposed methods are discussed in this paper.

Comprehensive tests were conducted on a longwall face in China to study mining-induced strata movement and fractures. The first and periodic weightings of the main roof were both captured by microseismic (MS) monitoring and support resistance recording and were 64 and 20 m, respectively. Based on the fracture detection through hydrological observations in a borehole, the roof is clearly divided into three zones: water-conductive zone, horizontal fracture, and continuous zone. Following the field investigation, an innovative finite-difference method (FDM) and distinct-element method (DEM) coupled numerical simulation method was proposed, which features simulating the progressive failure of the rock mass from intact to blocky. The numerical study shows that the proposed coupled method can give reasonable explanations for all in situ tests and handle the progressive caving of the rock layers, as well as the accompanying MS activities and response of the hydraulic supports. The proposed coupled method has advantages over a continuum or discrete method in coal mining simulation. Compared with a continuum method, the coupled method can simulate the deformation and collapse of the overburden strata, whereas the continuum method is only applicable before the first weighting of the rock layer, after which it gives wrong results. In contrast, compared with a discrete method where the blocks are factitiously set before and their sizes are mandatory, the coupled method allows the blocks to form automatically based on the failure state of the rock mass.

Drained torsional shear tests were performed on hollow cylindrical intact soft clay specimens to investigate the anisotropic deformation, strength, and noncoaxial behavior in general stress states involving variations in both the magnitude and direction of the principal stresses. Tests were conducted by maintaining a fixed principal stress direction α relative to the vertical direction while keeping the intermediate principal stress coefficient b constant. It was found that the magnitudes of the radial strain ε_r were dependent on the b value, and the magnitudes of circumferential strain ε_θ were dependent on the α angle. The influence of b and α on the relationship between volumetric strain and shear stress was less significant. The stress paths for all α angles at b = 0.5 were determined under various stress levels. Furthermore, the experimental results showed that the noncoincidence between the directions of strain increment and stress existed in clay, and the noncoaxiality was influenced by both intermediate principal stress and principal stress direction.

A series of triaxial creep tests were carried out on fractured limestone specimens under multilevel loading and unloading cycles to capture elastoviscoplastic strain components. A new data processing algorithm was proposed to analyze the experimental data, determine the instantaneous elastic and instantaneous plastic strain components and the viscoelastic and viscoplastic strain components from the total strain, and separate the viscoelastic and viscoplastic strain curves from the total creep strain curve. The instantaneous elastic strain, instantaneous plastic strain, viscoelastic strain, and viscoplastic strain versus deviatoric stress relationships are highly nonlinear. The proportion of the viscoplastic strain component in the total creep strain increases with increased deviatoric stress. On the basis of the experimental results, a nonlinear elastoviscoplastic (EVP) creep constitutive model was proposed by connecting a Hooke body, a parallel combination of Hooke and St. Venant bodies, a Kelvin body, and a generalized Bingham body. The proposed EVP creep model can describe both the loading and unloading creep behavior precisely. The creep model curves agree very well with the experimental results and give a precise description of the full stages of creep, especially the tertiary creep stage. Moreover, the variation law of the creep parameters is also supported by the experimental observations. So the validity and great practical potential of the proposed EVP creep model are shown well.

The shear response of rock joints under bolt reinforcement is important for supporting the design of a jointed rock mass. This paper aims to investigate the shear behavior of bolted rock joints with different roughness. Direct shear tests on both bolted and unbolted artificial joint specimens were conducted under various normal stresses. Before shear tests, three kinds of roughnesses for joints were created by split tests, and the roughnesses were quantitatively evaluated by a three-dimensional morphology scanner. After shear tests, the results showed that the bolt deformation decreased with increasing normal stress and increased with increasing roughness. Furthermore, the shear stiffness of the bolted joint specimens was, without exception, greater than that of unbolted specimens. Compared with those of unbolted specimens, the peak shear strength and the residual shear strength of bolted joint specimens with different roughnesses were improved to different degrees. Using the Coulomb failure criterion to linearly fit the shear strength and the applied normal stress, the equivalent cohesion and friction angle of the bolted joint specimens were found to have evident improvements compared with those of unbolted joint specimens. The increment rate of the equivalent cohesion of the bolted specimens increased with increasing roughness, whereas that of the equivalent friction angle decreased with increasing roughness.

This paper presents closed-form expressions for rocking spring stiffnesses and coupling interaction rotational spring stiffnesses for a set of closely spaced footings. Substructuring is used to derive analytically the exact reduced order spring models of the system. The stiffness coefficients of this reduced order model are determined by using (1) an extended, novel application of Boussinesq’s surface displacement of a point-loaded half-space and (2) an empirically derived formulation that makes use of both finite-element and experimental results. Further validation suggests that, within the scope of epistemic uncertainty present in the physical world, the interaction formulas between two footings are sufficient for more general multifooting interaction cases.

Rock-like materials generally manifest both strain-softening and pressure-dependent effects. The critical plastic strain corresponding to the residual state also increases with the confining pressure. In this case, all of the mechanical property parameters are the functions of plastic strain and confining pressure. The elastoplastic coupling strain-softening model (EPCSS) is established by dividing the elastic and plastic surrounding rock of circular openings into m and n annuli, respectively. When the range of each annulus is significantly small, the surrounding rock can be considered as isotropic and uniform. Then, the elastoplastic coupling analytical solutions can be derived by representing the material properties of each annulus with the corresponding value at each annulus’s outer radius for both Mohr–Coulomb (M-C) and Hoek–Brown (H-B) rock masses, respectively. Based on the continuum conditions of adjacent annuli, the equations for determining the radius of each annulus were established. Finally, the solutions were validated by strain-softening rock mass, and examples of EPCSS rock mass were further studied.

Because of the change in hydraulic gradient of a fully or partially submerged rock slope caused by a drop in the external or internal water level, enhanced seepage forces may lead to slope instability and collapse. Based on the Hoek-Brown failure criterion, a three-dimensional stability analysis on submerged rock slopes was conducted using the kinematic approach. Emphasis was given to applications of the equivalent Mohr-Coulomb parameters method and the generalized tangent technique and the adoption of the Hoek-Brown failure criterion to the stability analysis. The accuracy of the two methods for safety factor calculation was investigated and compared with the computational results. Stability analysis of a rock slope subjected to different hydraulic conditions was performed using the generalized tangential technique, and safety factors are presented in the form of stability charts for convenient practical use. The obtained results demonstrate the applicability of the nonlinear Hoek-Brown failure criterion in rock slope stability analysis. It was found that an increase in water level differences reduces slope stability, and different water drawdown conditions lead to various changing rules for obtained safety factors.

An important aspect in an underground radioactive waste disposal project is the characterization of hydrogen production due to steel container corrosion and its migration in the initially water-saturated host rock. For the French repository project, gas injection tests into initially saturated samples of the Callovo-Oxfordian (COx) claystone were conducted to mimic these phenomena. They showed an important expansion of the tested samples. In this paper, a series of hydromechanical simulations were performed, within the framework of two-phase flow in deformable porous media, to explain both hydraulic and mechanical responses of the material in those experiments. The analysis of experimental and numerical results suggested the simultaneous advance of gas and damage fronts through the sample. The increasing gas pressure led to the decay of the material rigidity and, consequently, to the alteration of the hydraulic properties, particularly the permeability. Simultaneously, the mechanical damage caused a significant expansion of the tested COx samples. The good agreement between the experimental and numerical results supported these explanations and should lead, in future works, to a more complete hydromechanical model aiming to simulate the problem of hydrogen production and migration.

The consolidation process of a stone-column reinforced foundation under partially drained boundaries was investigated on the basis of a newly developed consolidation theory. An analytical solution was created using a separation of variables technique and the theory of an eigenvalue problem. In this paper, a solution from the literature was referenced as a special case of the solution. Computational examples are presented and discussed. In the situation of a sand blanket layer on the top of the foundation, the solution that assumes a completely pervious top boundary tends to overestimate the consolidation speed, and the solution presented in this paper is more suitable for a calculation under this condition. The study on the consolidation process of the foundation under partially drained boundaries shows that the consolidation process is more rapid if the disturbance to the surrounding soil during the installation of the stone column is relatively less pronounced; choosing a stone column with a high oedometric modulus is advantageous for the consolidation of the foundation. Finally, the sensitivity to the effects of the variation of different parameters on the consolidation process was also analyzed.